Every transaction in LessAccounting should be associated with a financial account-- whether it's an account that auto imports such as from your bank or credit card, or one that you manage manually like a cash box account.
Sometimes you might have what we call "accountless" transactions, or transactions not tied to a financial account. This can happen if you manually create a transaction but don't designate what account the money belongs to, or if you delete the financial account completely but chose to keep the transactions in our app.
You're going to want to either delete these transactions, or properly assign them to the appropriate account.
Here's more info on finding these transactions fast: http://lessaccounting.groovehq.com/knowledge_base/topics/resolve-possible-duplicates